1. Field of the Invention
The present invention relates to a wafer pressurizing apparatus for applying an urging force to a wafer, and more particularly to such an apparatus suitable for use with a wafer polishing apparatus such as a chemical mechanical polishing (CMP) apparatus.
2. Description of the Related Art
In the past, a CMP apparatus has hitherto been known as this kind of wafer pressurizing apparatus. The CMP apparatus is generally constructed such that a wafer on a surface plate is carried by a carrier which is supported by a carrier shaft operatively connected with an upper cylinder. The carrier shaft is urged downwardly by means of the upper cylinder so as to push down the carrier whereby the wafer carried by the carrier is urged against a surface of the surface plate while the carrier is driven to rotate on its own axis of rotation and at the same time the surface plate is being rotated by a drive means such as an electric motor.
In a wafer polishing process using such a CMP apparatus, the magnitude of a polishing or urging force to be applied to the wafer during the polishing operation is very important from the point of view of polishing accuracy. That is, the polishing accuracy largely depends upon the polishing or urging force on the wafer.
Thus, a CMP apparatus has been proposed which is capable of maintaining the polishing force at a desired value by use of a pressure sensor such as a load cell.
FIG. 13 schematically illustrates a typical example of such a CMP apparatus. As clearly seen from this figure, this apparatus is equipped with a load cell LS which is disposed between a carrier shaft 110 mounted on a carrier 100 supporting a wafer 200 on a surface plate 210 and a piston rod 310 of a cylinder 300 which is adapted to push down the carrier shaft 110.
With this arrangement, the load cell LS senses the pressure or urging force of the cylinder 300 which is assumed as a polishing force actually applied to the wafer 200.
When the sensed pressure or force is different from a predetermined pressure value, the urging force of the cylinder is adjusted so as to maintain the sensed pressure at the predetermined pressure value.
In fact, however, the weights of the carrier shaft 110 and the carrier 100 in addition to the urging force of the cylinder 300 are applied to the wafer 200 as a wafer urging force. As a result, the sensed pressure value of the load cell LS generally differs from the actual polishing force.
Accordingly, the above-mentioned conventional CMP apparatus involves relatively large errors in controlling the polishing force applied to the wafer 200, thus impairing the polishing accuracy.
In view of the above, another type of CMP apparatus has been proposed which has a load cell LS provided within a carrier.
FIG. 14 is a cross-sectional view schematically illustrating essential portions of this CMP apparatus. In this apparatus, the carrier 100 includes a housing 101 supporting a wafer 200, and a flange 102 mounted on the housing 102 for movement in a vertical direction, with the load cell LS being disposed at a location between the housing 101 and the flange 102.
With the CMP apparatus as constructed above, the pressure or urging force due to the weights of the carrier shaft 110 and the carrier 100 in addition to the pressure force of the cylinder 300 can be sensed by the load cell LS, so that the pressure value thus sensed by the load cell LS becomes substantially equal to the actual polishing force applied to the wafer 200.
The CMP apparatus shown in FIG. 14, however, involves the following problems.
The carrier 100 is driven to rotate about its carrier shaft 110 while urging the wafer 200 against the rotating surface plate 210, so that in addition to a vertical force V, a rotational force R due to the self rotation of the carrier 100 and a horizontal force H due to the rotating surface plate 210 are applied to the load cell LS.
Consequently, the load cell LS senses the rotational force R and the horizontal force H in addition to the vertical force V which is to be sensed as the polishing force applied to the wafer 200, thus giving rise to potential errors in the control of the polishing force.
Moreover, application of the rotational force R and the horizontal force H to the load cell LS might damage the load cell LS.